Stress and Strain Measurement
Strain, e, is a dimensionless response to stress expressed as a fraction e=ΔL/Lo where Lo is the original length of the object and ΔL is the change in length of the object when stress is applied. Stress, s, is a measure of force per unit area given by F/A where F is the force being applied and A is the area it is being applied to. Because stress cannot be measured directly in practice, strain is measured instead. The stress in an object is related to the strain by the Young's Modulus, E, which is given by the following relationship:
  E  =            s      /              e        el              =                            (                      F            /            A                    )                          (                      Δ            ⁢                                                  ⁢                          L              /                              L                o                                              )                    =              stress        ⁢                  /                ⁢                  strain          .                    
Knowing the Elastic Modulus of a given material, the stress in the material can be determined by measuring the strain. Traditionally, stress and strain measurements have been accomplished by a number of different methods. Some of these methods are described below:
Strain Gages
Strain gages are small electronic devices that measure strain through a change in resistance. The resistance, R, of a wire is a function of the size of the wire as well as of the material as follows:
      R    =          ρ      ⁡              (                  L          A                )              ,where L is the length of the wire, A is the cross-sectional area of the wire, and ρ is electrical resistivity, a property of the material. As the length of the wire L increases and the cross-sectional area A decreases, the resistance R increases. This property can be exploited to measure strain with a strain gage. By measuring the increase in resistance of a length of a thin wire attached to a part, the strain in the part can be determined and the stress calculated.Fiber Optics
Fiber-optics can be used to measure stress and strain by detecting the change in length of an optical fiber. In theory, the operation of a fiber-optic strain gage is similar to the operation of a strain gage that measures change in resistance. In the case of a fiber-optic strain gage, a change in the transmissibility of light is being measured. Fiber-optic strain gages possess the same disadvantages as standard strain gages: they are difficult to apply and require external circuitry or instruments to interpret the signal.
Because of the disadvantages and the complexity of strain gages, brittle lacquer, and fiber-optics, these techniques for measuring stress and strain are typically used only at the product development stage for high-value products such as aircraft parts. Production parts and structures such as bridges and buildings generally do not come with built-in strain gages for monitoring stresses and strains, although this might be desirable in some cases. For example, monitoring the stresses and strains in a bridge or overpass could be useful for ensuring the safety of that structure. However, the cost of existing monitoring methods is prohibitive for widespread deployment into many production applications and civil engineering structures. Existing techniques of detecting stress and strain are expensive enough to make them somewhat prohibitive even on prototypes at the product development stage.
Brittle Coatings
Brittle lacquer is a brittle coating that cracks easily under tensile strain. The lacquer is applied to the unstressed part. When the part is stressed, the brittle lacquer cracks, starting at the areas of highest strain. Brittle lacquer is difficult to work with and does not provide a quantitative measure of the stress and strain. As such, the brittle lacquer method can only indicate which areas of a part are experiencing stress and strain. Also, only one test is possible with a given application of brittle lacquer. Once the brittle lacquer has cracked, the coating must be stripped off and reapplied for subsequent tests. One supplier of brittle lacquer coatings is StressCoat Inc. of Upland, Calif.
A similar type of coating that cracks under strain and can thus be used to detect strain is disclosed by Ifju et al (U.S. Pat. No. 6,327,030). Ifju's coating is luminescent and changes in strain cause cracks that can be seen because of the different properties in how the coating luminesces. The problem with this type of coating as with all brittle coatings is they are difficult to apply and use. The coating is only good for one test, and are typically not suitable for use in production parts and structural monitoring applications where environmental and corrosion protection are required.
Photoelastic Techniques
Photoelastic techniques are optical techniques for detecting stress and strain that make use of the photoelastic properties of certain materials. The speed of propagation of light in transparent materials is generally slower than in a vacuum or in air. The ratio of the speed of light in a given material to the speed of light in a vacuum is called the index of refraction of that material. In homogeneous materials, the index of refraction is constant regardless of the direction of propagation or plane of vibration of the light. In other materials, strain in the material causes the index of refraction to change depending on the direction of propagation of light. These materials, which can be optically isotropic when unstrained, become optically anisotropic when strain is present.
Materials that become optically anisotropic when stressed are known as photoelastic materials. The change in index of refraction relative to index axis in the material can typically be related to the stress and strain in the material by observing and quantifying the photoelastic effect. The photoelastic effect is caused by alternately constructive and destructive interference between light rays that have undergone relative retardation, or phase shift, in the stressed photoelastic material. When illuminated with polarized light and viewed through a polarizing filter, fringe patterns become visible in the photoelastic material that reveal the overall stress and strain distribution in the part and show the locations and magnitudes of the stresses and strains in the part. A person skilled in the art of photoelastic analysis can interpret and measure these patterns.
Photoelastic Coatings
Photoelastic coatings have traditionally only been used for laboratory testing or prototype testing because of the cost of the coatings, the difficulty of applying the coatings, and the unsuitability of the coatings for production components or for applying to structures in the field. Photoelastic coatings are available from companies such as Measurements Group (http://www.vishay.com/company/brands/measurements-group/) in sheet form, and also in a liquid plastic form that is cast onto prototype parts and then bonded on using adhesive. Both these types of coatings are cost and labour-intensive to apply, and are not well suited for complex parts, large parts, or parts made in higher quantities.
Lam and Ellens disclose a method for applying a photoelastic coating using powder coating techniques (U.S. Pat. No. 6,650,405) that is low cost, easy to apply, and can be applied on parts with complex three-dimensional shapes. The application of a photoelastic coating using powder coating methods is much less expensive than applying of traditional photoelastic coatings. The low cost makes a powder coated photoelastic layer more suitable for application to parts that are made in higher quantities for field use. Once powder coated with the photoelastic layer, the parts and structures can then be inspected in service using photoelastic techniques to determine if any strain is present in the part or if any plastic deformation has occurred. This is a useful and low cost method of monitoring parts and can increase public safety by helping with early detection of failures before they become catastrophic.
However, the method of applying a photoelastic coating techniques as disclosed by Lam and Ellens in U.S. Pat. No. 6,650,405 suffers from a number of limitations, for example in its applicability to structural monitoring of larger structures in the field such as bridges, buildings, and larger aerospace components. One limitation is that powder coating is applied as a dry finely-divided solid powder and typically needs to be baked on by increasing the temperature of the powder and the part being coated to an elevated level (typically from 100° C. to 200° C.). This can be impractical or inconvenient for large parts such as bridge trusses and large beams because these parts may not fit into an oven, and also because the energy required to heat these parts up to the temperature required could be prohibitive. It may also not be practical to apply powder coating to some structures already in use, particularly if they are installed on a permanent basis in the field. Finally, some structural parts and components are made out of materials that cannot be heated to the elevated temperatures required for curing powder. For example, some types of alloys are subjected to heat treatments that can be affected if the part is subsequently heated to an elevated temperature. Applying a photoelastic layer to these parts using powder coating may not be practical or possible.
Non Destructive Testing
Non-Destructive Testing (NDT) methods are used to inspect structures to determine if they are structurally sound, or if failure is imminent. However, existing NDT techniques are typically expensive to apply and cannot detect certain types of failure such as plastic deformation and simple overloading that does not result in cracks. Existing NDT techniques include liquid penetrant tests, eddy-current tests, and X-Ray testing. NDT techniques described can typically detect voids or cracks, but cannot typically detect whether a structural component has been subjected to a stress that is too high, or whether it has experienced any plastic deformation.
Structural Monitoring
Structures such as buildings, bridges, airplanes, and other critical structures are prone to failure. Failure can happen with significant loss of life and property, as evidenced by the recent collapse of the I-35 bridge in Minnesota. As such, structural monitoring is becoming an even more important field and can have a significant positive impact on public health and safety.
Prior art approaches to structural monitoring typically involve expensive monitoring devices and expensive monitoring systems. A prior art system that involves the use of a linear transducer is disclosed by Arms (U.S. Pat. No. 6,588,282). The transducer has two components that move relative to one another, and a clamping mechanism is used to prevent the shortening of the relative strain between the two components. This system has disadvantages because of the moving parts involved, and the precise orientation required for those moving parts to work. Also, the system requires an electronic system to detect the signal. These factors combine to make this system expensive and less suitable for large-scale deployment for structural monitoring.
Another prior-art approach to structural monitoring is disclosed by Brennen (U.S. Pat. No. 6,928,881). Brennen's approach involves the use of strain gauges mounted in a housing along with instrumentation that records and stores the stress levels experienced by the structure. While this approach can give an accurate time history of stresses experienced by a structure, it is prohibitively expensive because of the use of on-board monitoring and storage. The collection of all the historical stress data is excessive and unnecessary. The regular storage and retrieval of all the data is expensive and requires overly-frequent visits to perform downloads. Also, because data will only be recorded at a certain sampling rate, it is possible with Brennen's system to miss key events such as stress strain peaks. This could lead to false conclusions regarding the safety of the structure because key peak events have been missed. Increasing the sampling rate to try to capture these peak key events means more data needs to be stored. Reducing the sampling rate in order to reduce the amount of data stored only makes the problem of missing key events worse.
Bilder et al, (U.S. Pat. No. 5,534,289) discloses a method of structural crack detection that involves the use of microcapsules in a coating that will burst and change the colour of a second coating layer. The coatings also provide environmental protection for the structural part. This method however can only detect cracks, and the requirement of microcapsules containing coloured dyes makes the method difficult and expensive.
In summary, prior art stress and strain measurement and non-destructive testing techniques suffer from a number of problems. In particular, current solutions are too expensive, not adequate, or not capable of monitoring structural components such as bridges, buildings, and components such as aircraft landing gear in a low-cost manner that allows an inspector to determine whether the structure has experienced excessive stress strain levels or not.
Also, existing solutions for monitoring stress and strain in structural components are not well adapted for use in the field. Monitoring of stresses and strains in the field using existing instruments and methods typically requires constant monitoring and logging of data to capture the stress strain history of the structure and part. Current solutions are problematic and expensive and do not allow for periodic low-cost inspections of structural parts and components without requiring constant monitoring or monitoring equipment to be present at all times. There is need for a device that is relatively inexpensive to manufacture, easy to install, that allows for easy inspections of structures and parts in the field to determine information on the stress strain experienced by that part.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.